Sunday, September 22, 2013

The development of the photoelectric

The development of the photoelectric photometer allowed very precise measurements of magnitude at multiple wavelength intervals. In 1921 Albert A. Michelson made the first measurements of a stellar diameter using an interferometer on the Hooker telescope.[31]
Important theoretical work on the physical structure of stars occurred during the first decades of the twentieth century. TOSHIBA Satellite L300 Laptop Keyboard
In 1913, the Hertzsprung-Russell diagram was developed, propelling the astrophysical study of stars. Successful models were developed to explain the interiors of stars and stellar evolution.Cecilia Payne-Gaposchkin first proposed that stars were made primarily of hydrogen and helium in her 1925 PhD thesis.[32] TOSHIBA Satellite A215-S4807 Laptop Keyboard
The spectra of stars were further understood through advances in quantum physics. This allowed the chemical composition of the stellar atmosphere to be determined.[33]
With the exception of supernovae, individual stars have primarily been observed in our Local Group of galaxies,[34] and especially in the visible part of theMilky Way (as demonstrated by the detailed star catalogues available for our galaxy).[35] HP G72-b67CA Laptop Keyboard
But some stars have been observed in the M100 galaxy of the Virgo Cluster, about 100 million light years from the Earth.[36] In the Local Supercluster it is possible to see star clusters, and current telescopes could in principle observe faint individual stars in the Local Cluster[37] (see Cepheids). DELL Latitude D620 Laptop Keyboard
However, outside the Local Supercluster of galaxies, neither individual stars nor clusters of stars have been observed. The only exception is a faint image of a large star cluster containing hundreds of thousands of stars located at a distance of one billion light years[38]—ten times further than the most distant star cluster previously observed. HP 597635-001 Laptop Keyboard
The concept of the constellation was known to exist during the Babylonian period. Ancient sky watchers imagined that prominent arrangements of stars formed patterns, and they associated these with particular aspects of nature or their myths. Twelve of these formations lay along the band of the ecliptic and these became the basis of astrology.[39] ACER Aspire 5742 Laptop Keyboard
Many of the more prominent individual stars were also given names, particularly with Arabic or Latin designations.
As well as certain constellations and the Sun itself, individual stars have their own myths.[40] To the Ancient Greeks, some "stars", ACER Aspire 5336 Laptop Keyboard
known as planets (Greek πλανήτης (planētēs), meaning "wanderer"), represented various important deities, from which the names of the planets Mercury, Venus, Mars, Jupiter andSaturn were taken.[40] (Uranus and Neptune were also Greek and Roman gods, but neither planet was known in Antiquity because of their low brightness. Their names were assigned by later astronomers.)  HP 605344-001 Laptop Keyboard
Circa 1600, the names of the constellations were used to name the stars in the corresponding regions of the sky. The German astronomer Johann Bayer created a series of star maps and applied Greek letters as designations to the stars in each constellation. Later a numbering system based on the star's right ascensionwas DELL PVDG3 Laptop Keyboard
invented and added to John Flamsteed's star catalogue in his book "Historia coelestis Britannica" (the 1712 edition), whereby this numbering system came to be called Flamsteed designation or Flamsteed numbering.[41][42]
The only internationally recognized authority for naming celestial bodies is the International Astronomical Union (IAU)[43]  SAMSUNG N140 Laptop Keyboard
A number of private companies sell names of stars, which the British Library calls an unregulated commercial enterprise.[44][45] However, the IAU has disassociated itself from this commercial practice, and these names are neither recognized by the IAU nor used by them.[46] One such star naming company is the International Star Registry, which, TOSHIBA Mini NB 505-SP0160 Laptop Keyboard
during the 1980s, was accused of deceptive practice for making it appear that the assigned name was official. This now-discontinued ISR practice was informally labeled a scam and a fraud,
Stars are formed within extended regions of higher density in the interstellar medium, although the density is still lower than the inside of a vacuum chamber. SONY VAIO VGN-N31S/W Laptop Keyboard
These regions are called molecular clouds and consist mostly of hydrogen, with about 23–28% helium and a few percent heavier elements. One example of such a star-forming region is the Orion Nebula.[55] As massive stars are formed from molecular clouds, they powerfully illuminate those clouds. They also ionize the hydrogen, creating an H II region. HP Pavilion dv6-3217cl laptop keyboard
All stars spend the majority of their lives as main sequence stars, fueled primarily by the nuclear fusion of hydrogen into helium within their cores. However, stars of different masses have markedly different properties at various stages of their lives. The ultimate fate of more massive stars is different from that of less massive stars, as is their luminosity and the impact they have on their environment. SONY VAIO VGN-NW15G/S laptop keyboard
Therefore, stars are often grouped by mass. Very low mass stars with masses below 0.5 solar masses do not enter the asymptotic giant branch (AGB) but evolve directly into white dwarfs. Low mass stars (including the Sun) with a mass above about 0.5 and below about 1.8–2.2 solar masses (depending on composition) DELL PK130CW1A08 laptop keyboard
do enter the AGB, where they develop a degenerate helium core.Intermediate-mass stars undergo helium fusion and develop a degenerate carbon-oxygen core. Massive stars have a minimum mass of 7–10 solar masses, but this may be as low as 5–6 solar masses. These stars undergo carbon fusion, with their lives ending in a core-collapse supernova explosion. ASUS A6J laptop keyboard
The formation of a star begins with gravitational instability within a molecular cloud, caused by regions of higher density often triggered by shock waves from nearby supernovae (massive stellar explosions), the collision of different molecular clouds, or the collision of galaxies (as in a starburst galaxy). SONY VAIO VGN-CR21SR laptop keyboard
Once a region reaches a sufficient density of matter to satisfy the criteria for Jeans instability, it begins to collapse under its own gravitational force.
As the cloud collapses, individual conglomerations of dense dust and gas form what are known as Bok globules. ACER TravelMate 8210 laptop keyboard
As a globule collapses and the density increases, the gravitational energy is converted into heat and the temperature rises. When the protostellar cloud has approximately reached the stable condition of hydrostatic equilibrium, a protostar forms at the core.[58] These pre–main sequence stars are often surrounded by aprotoplanetary disk and powered mainly by the release of gravitational energy. TOSHIBA Satellite L775D-S7340 laptop keyboard
The period of gravitational contraction lasts about 10–15 million years.
Early stars of less than 2 solar masses are called T Tauri stars, while those with greater mass are Herbig Ae/Be stars. These newly born stars emit jets of gas along their axis of rotation, which may reduce the angular momentum of the collapsing star and result in small patches of nebulosity known as Herbig–Haro objects.[59][60] SONY VAIO PCG-FR130 laptop keyboard
These jets, in combination with radiation from nearby massive stars, may help to drive away the surrounding cloud from which the star was formed.[61]
Early in their life, T Tauri stars follow the Hayashi track--they contract and decrease in luminosity while remaining at roughly the same temperature. TOSHIBA Satellite C655-S9520D laptop keyboard
Less massive T Tauri stars follow this track to the main sequence, while more massive stars turn onto the Henyey track.
Stars spend about 90% of their lifetime fusing hydrogen into helium in high-temperature and high-pressure reactions near the core. Such stars are said to be on the main sequence and are called dwarf stars.  HP Pavilion DV7-3060us laptop keyboard
Starting at zero-age main sequence, the proportion of helium in a star's core will steadily increase, the rate of nuclear fusion at the core will slowly increase, as will the star's temperature and luminosity.[62] The Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion (4.6 × 109) years ago.[63]  HP G62-b12SL laptop keyboard
Every star generates a stellar wind of particles that causes a continual outflow of gas into space. For most stars, the mass lost is negligible. The Sun loses 10−14 solar masses every year,[64] or about 0.01% of its total mass over its entire lifespan. However, very massive stars can lose 10−7 to 10−5 solar masses each year, significantly affecting their evolution.[65] TOSHIBA NB100 laptop keyboard
 Stars that begin with more than 50 solar masses can lose over half their total mass while on the main sequence.
The duration that a star spends on the main sequence depends primarily on the amount of fuel it has to fuse and the rate at which it fuses that fuel, i.e. its initial mass and its luminosity. TOSHIBA Satellite L200 laptop keyboard
For the Sun, its life is estimated to be about 10 billion (1010) years. Massive stars consume their fuel very rapidly and are short-lived. Low mass stars consume their fuel very slowly. Stars less massive than 0.25 solar masses, called red dwarfs, are able to fuse nearly all of their mass as fuel while stars of about 1 solar mass can only use about 10% of their mass as fuel. Lenovo 0A62075 laptop keyboard
The combination of their slow fuel consumption and relatively large usable fuel supply allows stars about 0.25 times the mass of the Sun to last for about one trillion (1012) years according to stellar evolution calculations, while the least-massive hydrogen-fusing stars (0.08 solar masses) will last for about 12 trillion years.[67] TOSHIBA Satellite L750-ST4N02 Laptop Keyboard
At the end of their lives, red dwarfs simply become dimmer and dimmer.[2] However, since the lifespan of such stars is greater than the current age of the universe (13.8 billion years), no stars under about 0.85 solar masses[68] are expected to have moved off of the main sequence. ACER Aspire 5742 Laptop Keyboard
Besides mass, the elements heavier than helium can play a significant role in the evolution of stars. In astronomy all elements heavier than helium are considered a "metal", and the chemical concentration of these elements is called the metallicity. The metallicity can influence the duration that a star will burn its fuel, control the formation of magnetic fields[69] and modify the strength of the stellar wind.[70] HP 605344-001 Laptop Keyboard
Older, population II stars have substantially less metallicity than the younger, population I stars due to the composition of the molecular clouds from which they formed. Over time these clouds become increasingly enriched in heavier elements as older stars die and shed portions of theiratmospheres. SAMSUNG N150 Laptop Keyboard
As stars of at least 0.4 solar masses[2] exhaust their supply of hydrogen at their core, their outer layers expand greatly and cool to form a red giant. In about 5 billion years, when the Sun enters this phase, it will expand to a maximum radius of roughly 1 astronomical unit (150 million kilometres), 250 times its present size. As a giant, the Sun will lose roughly 30% of its current mass.[63][71 HP G72-a10SV Laptop Keyboard
In a red giant of up to 2.25 solar masses, hydrogen fusion proceeds in a shell surrounding the core.[72] Eventually the core is compressed enough to start helium fusion, and the star now gradually shrinks in radius and its surface temperature increases. For larger stars, the core region transitions directly from fusing hydrogen to fusing helium.[4] DELL Vostro 1015 Laptop Keyboard
After the star has consumed the helium at the core, fusion continues in a shell around a hot core of carbon and oxygen. The star then follows an evolutionary path that parallels the original red giant phase, but at a higher surface temperature.
During their helium-burning phase, very high mass stars with more than nine solar masses expand to form red supergiants. SONY VAIO VGN-N31S/W Laptop Keyboard
Once this fuel is exhausted at the core, they continue to fuse elements heavier than helium.
The core contracts until the temperature and pressure are sufficient to fuse carbon (see carbon burning process). This process continues, with the successive stages being fueled by neon (see neon burning process), SONY VAIO VGN-CR150F Laptop Keyboard
oxygen (see oxygen burning process), and silicon (see silicon burning process). Near the end of the star's life, fusion continues along a series of onion-layer shells within the star. Each shell fuses a different element, with the outermost shell fusing hydrogen; the next shell fusing helium, and so forth.[73] DELL Inspiron 9300 Laptop Keyboard
The final stage is reached when a massive star begins producing iron. Since iron nuclei are more tightly bound than any heavier nuclei, any fusion beyond iron does not produce a net release of energy—the process would, on the contrary, consume energy. Likewise, since they are more tightly bound than all lighter nuclei, energy cannot be released by fission.[72] DELL Latitude D505 Laptop Keyboard
In relatively old, very massive stars, a large core of inert iron will accumulate in the center of the star. The heavier elements in these stars can work their way to the surface, forming evolved objects known as Wolf-Rayet stars that have a dense stellar wind which sheds the outer atmosphere. HP Pavilion DV3000 Laptop Keyboard

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